Fire alarm with ionization chamber

ABSTRACT

In the alarm, a circuit generates a quiescent ionization current between two electrodes in a chamber having a radioactive source. The chamber admits ambient air. Combustion gases from smokeless fires cause additional ionization current. The circuit establishes the quiescent ionization current at a value close to the additional ionization current. Amplifying means in the circuit respond to increases and decreases in the ionization current so as to indicate existance of various types of fires. An alarm device responds to the amplifier. The quiescent current may be within the order of magnitude of the additonal ionization current. Preferably the amplifying means responds to increases and decreases in ionizaton current with an equal quiescent ionization current in a reference chamber.

7/1970 Lampart et all 340/237 S eyersdorf etal. at. 5, 1974 [54] P :1 3? IONHZATION FOREIGN PATENTS OR APPLICATIONS 1,088,976 10/1967 Great Britain 250/435 D [75] Inventors: Hartwig Beyersdorf; Lothar Rims,

both of Bad Oldesloe, Germany Primary ExaminerJohn w. Caldwell [73] As signee: Preussag AG lFeuerschutz, Bad Assistant Exammer namel Myer Oldesloe, Germany Attorney, Agent, or Firm--Toren & McGeady [22] Filed: May 13, 1971 [57] ABSTRACT 1 1 PP N05 142,892 In the alarm, a circuit generates a quiescent ionization current between two electrodes in a chamber having a [30] Foreign Application Priority Data radioactive source. The chamber admits ambient air. Ma 16 1970 German 2024116 Combustion gases from smokeless fires cause addi- A Germany 21 3821 tional ionization current. The circuit establishes the p y quiescent ionization current at a value close to the ad- [52] U 8 Cl 340/2 37 s 250/83 6 Fr ditional ionization current. Amplifying means inthe [51] hit Cl G08b'17 6 H0] j 59/28 circuit respond to increases and decreases in the ion- [58] Fie'ld s "El 256/43 5 D 44 ization current so as to indicate existance of various 250283 1 types of fires. An alarm device responds to the amplifier. The quiescent current may be within the order of [56] References Cited magnitude of the additonal ionization current. Prefer- I ably the amplifying means responds to increases and UNHED STATES PATENTS decreases in ionizaton current with an equal quiescent 3.22: g i' t:

ionization current in a reference chamber. 3,44 mirago 3 521 263 25 Claims, 13 lDll'a Figures PATENTEI] MR 5 I974 SHEEI 2 OF 5 sniff/ 25 27 31 JZBZS 2EL Z8 31 Fig.4

INVENTORS HHRTHIG BEYERSDDRF LOTHRR Rms BYI/ WWW HTHRMEYS FIRE ALARM WITH IONIIZATION CHAMBER BACKGROUND OF THE INVENTION This invention relates to fire alarms and particularly to fire alarms using ionization chambers.

In such alarms electrodes are arranged in an ionization chamber to which an electric voltage is applied. A radioactive source is used for generating an ionization current between the electrodes.

In one known device for detecting smoke particles or combustion products, the voltage appearing at an electrode is detected by applying the voltage to an indicating device with an amplifier. No bias is applied to the electrode, which is charged up electrost'atically by the smoke particles or combustion products flowing past the electrode. This device utilizes the fact that the smoke of substances that might be involved in a fire exhibits definite electrical charges which can induce voltages at the insulated electrode.

This device has the disadvantage that it may fail to respond when electrically charged smoke particles have been already gotten into the measuring chamber of the device are carried out again by a draft without giving up their electric charge and being used for producing a reading. The device may be furtherineffective in that it is relatively insensitive in the presence of small amounts of dust. Smoke particles which slowly float around without being accelerated by an electric field can settle on the electrode on top of the insulating layer of dust. These can no longer impart their charges to the insulated electrode with any degree of effectiveness and may not be able to impart their charges to the electrode at all.

One device for measuring the electric charge of atmospheric air utilizes a blower to drawair through a probe arrangement which forms an electric capacitor. The change in the charge of the probe arrangement which is caused by the electric charge of the air is automatically applied in a definite sequence to an amplifier, storage and. other auxiliary devices,-and via the latter to a measuring instrument. Measuring the very small values of electrical conductivity of the air involves connecting the electrodes of the probe arrangement to an accelerating voltage by means of an electrostatic capacitor. Such a device is not suitable for use as an indicating mechanism for smoke particles and combustion products. In particular it is not suitable for continuously monitoring a room because the periodic switching processes required are wasteful and expensive.

It has been known that current flowing in an ionization chamber decreases if fire gases enter. This decrease is explained by adsorption of gas ions on aerosol tone. In fact such fire alarms may-be unable to detect such fires altogether. Many ionization fire detectors also have the disadvantage of being insensitive to the reduction of ionization current resulting from dust accumulation, especially on the radioactive source.

It is an object of this invention to produce an ionization fire alarm for detecting smoke-forming as well as smokeless or low-smoke fires.

Another object of the invention is to improve fire alarms.

Still another object of the invention is to eliminate the deficiencies of prior fire alarm devices.

SUMMARY OF THE INVENTION According to a feature of the invention these objects are attained in whole or in part, in an ionization fire alarm wherein electrodes are arranged in an ionization chamber with at least one radioactive source, by applying a sufiicient voltage across the electrodes to generate an ionization current which is in the order of magnitude of the added ion currents that a smokeless fire would cause.

The invention is based upon the recognition that hot as well as cold flames, such as for example ether flames, are electrically conducting. This conductivity is caused by the chemical processes taking place during combustion. At that time free radicals are formed which serve as the carriers of a chain reaction. The non-neutral electrical particles formed in this manner are trans ported upward by the rising warm air. In smoke forming flames a pronounced recombination effect of the free ions and adsorption of the ions at the smoke aerosols takes place. Thus, when fire gases enter the ionization chamber only smoke particles are carried along. These cause a decrease of the ionization current in the measuring chamber.

The invention is also based on the recognition that in the presence of low-smoke or smokeless fires the number of recombinations of free ions of the flames is relatively low. Thus, many free ions enter the ionization chamber and contribute to amplification of reinforcement of the ionization current generated by the radioactive source. This amplification or reinforcement is measurable, only if, as in accordance with the invention, an ionization current flows in the quiescent state of the ionization chamber which is in the order of magnitude of the ion current that would be added in the ionization chamber by smokeless fires.

In contrast to ionization fire alarms hitherto known, the ionization current generated by the radioactive source is therefore extremely low. An ionization current satisfying these requirements is obtained if, according to one embodiment of the invention, the radioactive source exhibits a radioactivity of 0.1 microcuries or less.

According to another feature of the invention, one of the electrodes in the chamber surrounds the other at least partially and forms passage holes for the passage of gases or smoke particles or both.

According to another feature of the invention the potential of the outer electrode is more positive than that of the inside electrode.

According to yet another feature of the invention the radioactive source is arranged on the outer electrode.

According to still another feature of the invention an amplifier is connected to the negative electrode.

By virtue of these features the charge carriers carried along with the smoke particles, and among the free particles, are always positive charge carriers which furthermore generally predominate over the possibly present negative charge carriers. Thus the positive potential of the outer electrode prevents positive charges already at the outer electrode from being given off. In such cases the negative electrode, located inside, has a certain accelerating effect. Thus the arrangement of the radioactive source at the outer electrode improves the effectiveness of the ionization fire alarm according to the invention. According to another feature of the invention, a parameter characteristic of the dust accumulation on the radioactive source is monitored. In response to the parameter a fire alarm is prevented when the ionization current is decreasing slowly, or producing a dust alarm signal when a threshold value of the parameter is passed, or changing the resistance proportions in the current path through which the ionization current flows so as to make them equal to the original resistance proportions with no dust having yet collected on the source of radioactive radiation. According to this embodiment of the invention, the fire alarm is prevented in response to any combination of the beforementioned decrease, dust alarm signal production, and change.

These and other features of the invention are pointed out in the claims. Other objects and advantages of the invention will become known from the following detailed description when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram illustrating a fire alarm embodying features of the invention and including an ionization measuring chamber and a reference ionization chamber as well as an electric circuit arrangement therefor;

FIG. 2 is a schematic diagram of another fire alarm embodying features of the invention;

FIG. 3 is a schematic diagram illustrating another embodiment of FIG. 2; and

FIG. 4 is a schematic diagram illustrating still another embodiment of the fire alarm in FIG. 1.

FIG. 5 is a schematic diagram illustrating an ionization fire detector having means for preventing a fire alarm when the ionization current is slowly changing,

FIG. 6 is a schematic diagram showing parts of an ionization fire detector having means for producing a dust alarm signal when excessive dust accumulates on the radioactive source;

FIG. 7 is a graph illustrating the independence of the output voltage and the input voltage of an amplifier having threshold characteristics and used with the fire detector in FIG. 6;

FIG. 8 is a schematic diagram illustrating an ionization fire detector with means for presenting a fire alarm when the ionization current is slowly decreasing and for producing a dust alarm signal when excessive dust has collected on the radioactive source;

FIG. 9 is a schematic diagram showing details of an arrangement of an ionization fire detector in which a dust alarm signal is produced when the activity of the radioactive source falls below a threshold value;

FIG. 10 is a schematic and partially block diagram of an ionization fire detector in which increasing dust accumulation changes the resistance proportions in the current path through which the ionization current flows so as to make them equal to the original resistance proportions when no dust has yet collected on the source of radioactive radiation;

FIG. 11 is a schematic diagram illustrating details of another ionization fire detector in which a dust alarm signal is produced at a threshold value of dust accumulation on the radioactive source;

FIG. 12 is a schematic diagram illustrating parts of an ionization fire detector wherein decreasing dust accumulation changes the resistance proportions in the current path through which the ionization current flows so as to render them equal to the original resistance proportions when no dust has yet collected on the source of radioactive radiation; and

FIG. 13 is a schematic diagram illustrating another embodiment of an ionization fire detector in which a fire alarm is prevented when the ionization current is slowly decreasing because of dust accumulation.

DESCRIPTION OF PREFERRED EMBODIMENTS In FIG. 1 a measuring chamber 1 and a reference chamber 2, both shown schematically, are energized by a D-C voltage source not shown which applies a positive potential to a terminal 9 and a negative potential to a terminal 10. The chambers 1 and 2 are connected in series between the terminals 9 and 10. In the chambers 1 and 2 two outer electrodes 24 and 28 surround respective inner electrodes 25 and 27 so as to form respective measuring spaces 26 and 29 between them.

The positive potential at the terminal 9 and the negative potential at the terminal 10 places the outer electrode 24 of the measuring chamber 1 at a positive potential and the inner electrode 25 at a negative potential. Similarly the voltage source at the terminals 9 and 10 places the inner electrode 27 of the chamber 2 at a positive potential while the outer electrode 28 assumes a negative potential. Respective radioactive radiators 30 and 31 are located at the outer electrode 24 of the measuring chamber 1 and at the inner electrode 27 of the reference chamber 2.

An amplifying system designated 3, 4 and composed of a field-effect transistor followed by a transistor responds to the voltages appearing at the junction of the chambers l and 2 and the junction of a variable resistor 5 and a fixed resistor 6 which are connected in series between the terminals 9 and 10. In effect the chambers 1 and 2 as well as the resistors 5 and 6 form a bridge. The amplifying system 3, 4 measures the bridge on balance. On the other hand, the resistors 5 and 6 may be considered as biasing resistors which adjust the operating voltage of .the amplifier.

The resistor,.5 adjusts the bridge or the amplifier so that the amplifier output designated 11,12 exhibits half the operating voltage applied to terminals 9 and 10.

If smoke particles enter the measuring chamber 1, the voltage at the output 11,12 rises to the full operating voltage. However, if ions or free radicals enter the measuring chamber 1 the voltage appearing at the output 11,12 drops to zero.

In FIG. 2, the measuring chamber land the reference chamber 2 are again connected in series. However, here two amplifying systems 3 and 4 are provided with respective outputs 11 and 12. The amplifiers incorporated in the amplifying systems 3 and 4 are fieldeffect transistors followed by transistors. According to one embodiment of the invention these transistors are constructed as so called integrated MOS field-effect transistors. A voltage divider composed of resistors 5 and 6 is adjusted to apply a voltage at its junction to the amplifying system 3 such as to produce a voltage at the output 12. If a smokeless fire adds ions to the measuring chamber 1 the total ion current increases. This causes the measuring chamber to exhibit a low resistance state. The amplifier output 12 is then shifted to zero or nearly zero within a short time. This produces a fire alarm.

A voltage divider composed of resistors 7 and 8, of which the resistor 7 is variable, adjusts the output of the amplifying system 4 so that a near zero potential exists at the output 11. If smoke aerosols enter the measuring chamber 1 the ions generated by the radioactive radiator 30 attach themselves to the smoke particles. The ion current decreases. The measuring chamber then exhibits a resistance higher than before the entry of smoke aerosols. This produces a voltage at the output 11 of the amplifier 4. This again sets off a fire alarm.

In FIG. 3 two amplifying systems are again utilized. However, here the system 3 includes two MOS fieldeffect transistors 16 and 17 in cascade circuit. Two MOS field-effect transistors 19 and 20 form a cascade circuit in system 4. A cascade circuit of this type combines the advantages of source and gate circuits, namely the high gain as well as low noise of a source circuit and the high current stability of a gate circuit. A cascade stage is furthermore distinguished by good control properties. The control of the cascade is effected, as in a source or gate circuit, by reducing the operating slope. Variable tapped voltage dividers composed of resistors 15 and 21 adjust the gates of the stages 16 and 19. v 1

FIG. 4 illustrates an embodiment of the invention utilizing signal evaluation by means of an FET differential amplifier. Here an amplifying circuit 22 is composed of a field-effect transistor followed by a transistor. The field-effect transistor is connected to the junction of the chambers 1 and 2. Similarly an amplifying circuit 23 is composed of a field effect transistor followed by a transistor. The field effect transistor in the circuit 23 receives potential from the tap of a tapped voltage divider 115. The circuit 23 serves as the reference portion for the differential amplifier. A zener diode I4 stabilizes the input to the reference portion formed by the circuit 23. The signal outputs 11 and 12 of the differential amplifier are connected between two load resistors 26 and 27. A constant current regulator in the form of a zener diode 13 reduces the high sensitivity of the amplifier to temperature drift.

In FIGS. 1 through t the circuits including the radioactive radiators 30 and 311 are set to establish, in each chamber, an ionization current which is in the order of magnitude of the ionization current that is added by the gases from smokeless fires. According to one embodiment of the invention the radioactive source 30 has a maximum radioactivity of 0.1 microcuries. Preferably it has a far lower value. According to an embodiment of the invention the radioactive source 31 has a radioactivity equal to that of the radioactive source 30.

One of the advantages of devices embodying features of the invention, such as the devices in FIGS. 1 through d, resides in the ability of such devices to detect ions generated by thermal decomposition. This is particularly advantageous in the .case of thermal decomposition of plastics, especially where hydrochloric acid is split off from polyvinyl chloride. Such thermal decompositions can occur at temperatures which are well below the flare point of combustible substances. Thus noticeable emission of hydrochloric acid can occur at temperatures as low as 100 C in hard PVC. At 300 C percent of the hydrochloric acid may be liberated. When subject to moisture in the air the hydrochloric acid is dissociated. This causes acid gas to precipitate on accessible surfaces as very fine condensed droplets.

Continuous dissociation of PVC can cause irreparable corrosive damage to buildings and machinery.

Due to the particular ionization currents selected, fire alarms embodying features of the invention are capable of detecting free ions occurrring in such thermal decompositions without requiring the presence of an actual fire.

Fire alarms embodying features of the invention also turn out to be extremely advantageous because of the low radioactivity of the radioactive radiators. These are 0.1 microcuries or less to achieve the relatively small ionization current. The danger of radioactive contamination is considerably reduced thereby.

In FIGS. 1 through d the terminals 111 and 12 are connected to a suitable fire alarm apparatus, not shown, according to one embodiment of the invention. According to another embodiment of the invention the terminals 11 and 12 in FIGS. 1 through 4 are connected to a fire extinguishing apparatus.

. The invention contemplates the use of radiators 30 and 31 of less than 0.1 microcuries, for example 0.05 microcuries.

The voltage between the terminals 9 and 10 of FIGS. 1 to 4 is, according to one embodiment of the invention equal to 12 volts. The alarm device or extinguisher utilized in FIG.1, according to one embodiment of the invention, is connected between terminals 11,12, and 10, and is actuated when the voltage exceeds one threshold level or falls below a lower threshold level. The alarm device or extinguisher in FIGS. 2 to 4 is connected, according to one embodiment of the invention, between terminals 11 and 10 and between terminals 12 and 10. It is actuated on the one hand when the voltage across terminals 11 and It] exceeds one threshold level or the voltage between terminals 12 and 10 falls below a threshold level.

In the ionization fire detector shown in FIG. 5 there are diagrammatically represented a measurement chamber 1 and a reference chamber 2. They have outer electrodes 24 or 28 and inner electrodes 25 -or 27 which form measurement spaces 26 or 29 between them. The two chambers 1,2 are connected in series and are linked to a source of direct current (not shown) by way of clamps or terminals 9, 10. There is positive electrical potential at the outer electrode 24 of the measurement chamber 11, while there is negative electrical potential at the inner electrode 25. At the outer electrode 24 of the measurement chamber 1 and the inner electrode 27 of the reference chamber 2 there is in each case arranged a source 30 or 31 of radioactive radiation.

Connected to a point between the two inner electrodes 25, 27 is a field-effect transistor 32. Its drain electrode is connected to the positive clamp 9 via a resistor 33, and its source electrode is connected to the slider of a potentiometer 34. The potentiometer 34 is placed for voltage stabilization purposes parallel to a Zener diode 35 which is connected in series with resistor 36.

The potentiometer 3d is adjusted so that the input of differentiating amplifier 38, which is connected to the drain electrode of the field-effect transistor 32, en-

counters about half of the operating voltage which is applied to clamps 9, 10. When smoke particles enter the measurement chamber 1 the voltage at the input 37 rises to almost the same as the full operating voltage. On the other hand, when ions or free radicals enter into the measurement chamber 1 the voltage at input 37 will drop towards zero.

The differentiating amplifier 38 comprises an operational amplifier 39, a capacitor 40, which is connected between the input 37 and the inverted input of the operation amplifier 39, as well as a resistor 41 which is connected between the output of operational amplifier 39 and its inverted input. The noninverted input of operational amplifier 39 is connected to the negative clamp 10.

The output of the differentiating amplifier 38 is connected to a threshold amplifier 42. It comprises an operational amplifier 43, a resistor 44, which is connected between the output of differentiating amplifier 38 and the inverted input of the operation amplifier 43, two resistors 45, 46 which are connected as voltage dividers between the output of the operation amplifier 43 and the negative clamp 10, and a resistor 47 which leads from the connecting point between resistors 45, 46 back to the noninverted input of the operation amplifier 43. The output of the threshold amplifier 42 is connected to clamp or terminal 48. At terminal 48, a fire alarm signal indicating flames or smoke is produced only when the time differential of the ionization current, i.e., the velocity of its increase or decrease, which is determined by the differentiating amplifier 38, surpasses a certain threshold value. A reduction of the ionization current caused by dust accumulation within the measurement chamber 1 and especially on the source of radioactive radiation 30, takes place so slowly that v the differentiating amplifier 38 produces a signal which is below the threshold value of the threshold amplifier 32 so that no fire alarm signal appears at the terminal 48. Instead of the differentiating amplifier 38, differentiating units of a different kind known per se and instead of threshold amplifier 42 networks with threshold characteristics of different kinds known per se may be used.

In FIG. 6 the measurement chamber 1 and the reference chamber 2 are again connected in series. To a point between the chambers l, 2 there are connected as indicated by lead 49 means (not shown) for producing a fire alarm signal, for example the means illustrated in FIG. 5. Furthermore, there is connected to this point a field-effect transistor 50 which is connected in an analogous way to field-effect transistor 32 of FIG. 5, in a network connected between the clamps 9, 10 and formed by resistor 51, potentiometer 52, diode 53 and another resistor 54. The drain electrode of the field-effect transistor 50 is connected to the input of a threshold amplifier 55 which. analogous to the threshold amplifier 42 of FIG. 5, consists of an operational amplifier 56, an input resistor 57, a voltage divider potentiometer S8 and a return resistor 59. The threshold amplifier 55 produces at its output, which is connected to a clamp or terminal 60, a signal when the ionization current differs by a certain threshold value from the value prevailing in the measurement chamber 1 when no dust has yet collected on the source of radioactive radiation. The signalproduced at clamp 60 causes a visual or acoustic dust alarm signal or can be used for preventing a fire alarm.

The course of the output voltage 14,, of the threshold amplifierSS in dependence of its input voltage u is represented in FIG. 7. Up to a certain threshold value of the input voltage u the respective output voltage does not change. When the threshold value is reached the output voltage a at terminal 60 suddenly changes its sign.

The ionization fire detector in accordance with FIG. 8 combines essentially the measures in accordance with FIG. 5 and 6. The measurement chamber 1 is in this case connected in series not with a reference chamber but with a reference resistor in the form of a reversebiased diode 61. To the point between the measure- I ment chamber 1 and the diode 61 there is connected, as in FIG. 5, a field-effect transistor 32, which is connected between the clamps 9, 10 in a network consisting of resistor 33, potentiometer 34, diode 35 and another resistor 36. The drain electrode of the field-effect transistor 32 is again connected to the input 37 of differentiating amplifier 38, to the output of which a threshold amplifier 42 is connected. However, the signal channel formed by the field-effect transistor 32, the differentiating amplifier 38 and the threshold amplifier 32, serves in this case, unlike with the example of embodiment in accordance with FIG. 1, only for signalling smoke, i.e., for detecting a fast reduction of the ionization current. To this end the field-effect transistor 32 is adjusted by means of the potentiometer 34 so that at the input 37 there is a potential of almost zero. When smoke aerosols enter the measurement chamber 1 the flow of ions is reduced; from an electrical point of view, the measurement chamber 1 becomes-more highly resistant. Thereby a fire alarm signal is produced at the output of the threshold amplifier 42.

For detecting the ions produced in the case of a smokeless fire a separate signal channel is provided which is formed by another field-effect transistor 62.

This one is connected in an analogous way to the fieldeffect transistor 32 between the clamps 9, 10 in a network which is formed by resistor 63, potentiometer.64, diode 65 and another resistor 66. The fieldeffect transistor 62 is adjusted by means of the potentiometer 64 so that a certain voltage is normally applied to its drain electrode. When additional ions enter the measurement chamber 1, for example in the case of a smokeless fire, the total flow of ions increases, i.e., the measurement chamber 1 adopts for the circuit a condition of low impedance. The voltage at the drain electrode of the field-effect transistor 62 is regulated towards zero within very short time. This is again a fire alarm signal.

Both kinds of fire alarm signals start a multivibrator 67 which is connected to the outputs of both threshold value amplifier 42 and field-effect transistor 62. If smoke or ions, which have for example been generated by a smokeless fire, occur a fire signal is produced at its output which is connected to clamp 48.

Dust accumulation within measurement space 26 of measurement chamber 1, especially on the source of radioactive radiation 30, contrary to fires, can only cause a reduction of the flow of ions. Therefore, the threshold amplifier 55 which has already been described in conjunction with FIG. 6 is connected to the signal channel which serves for smoke detection, namely to the drain electrode of the field-effect transistor 32. When the ionization current falls below a threshold-value the threshold amplifier 55 produces, as

in FIG. 6 a dust alarm signal at its output which is connected to clamp 60.

When smoke enters into the measurement chamber 1 signals are produced at the clamp 48 as well as at the clamp 60. If desired, the dust alarm signal can be suppressed in this case by means well known in the art and not especially represented.

In FIG. 9 is represented the measurement chamber 68 of an ionization fire detector, which is similar to the measurement chamber 1 of the examples in accordance with FIGS. 5, 6, and 8. It has also an outer electrode 69, an inner electrode 70, a measurement space 71 formed between electrodes 69, 70 and a source of radioactive radiation 72. In this case, however, the inner electrode 70 has an annular form and has in its centre an opening through which a radiation detector 73 projects into the interior of measurement space 71, this detector being exposed to the radiation of the radioactive source 72. The intensity of radiation which is measured by the radiation detector 73 represents a parameter characteristic of the dust accumulation on the source of radioactive radiation 72, since the measured radiation intensity decreases with increasing dust accumulation on the source of radioactive radiation 72 and with the simultaneously increasing dust accumulation on the surface of the radiation detector 73 facing the source of radiation 72. For power supply of the radiation detector 73 and for pre-amplification of its output signals it is provided with an amplifier 74. The output i of amplifier 74 is connected to a threshold amplifier 75 which produces a dust alarm signal when the decrease in radiation power of the source of radiation surpasses a threshold value. With the same means it is also possible to prevent the production of a fire alarm signal.

In the example of embodiment represented in FIG. 10 the reference resistor which is connected in series with the measurement chamber 68 is formed by the reverse-biased diode 61 and a series-connected transistor 76; it could also be formed exclusively of the transistor 76. The output signal of amplifier 74 of the radiation detector 73 is fed to a controller 77. When the radiation power, as measured by the radiation detector 73, decreases due to dust accumulation caused, then the internal resistance of themeasurementchamber 68 is increased. The controller 77 increases accordingly the resistance value of the reference resistor, by reducing the conductivity of the transistor 76. By this means it is achieved that with increasing dust accumulation the resistance proportions in the current path through which the ionization current flows, i.e., the proportions between the internal resistance of the measurement chamber 68 and the reference resistor, are changed in accordance with the original resistance proportions with no dust having yet collected on the source of radiation 72. As a result of this arrangement the potential at the point between measurement chamber '68 and the reference resistor formed by diode 61 and the transistor 76 does not change even with increasing dust accumulation. The means for producing a fire alarm signal (not represented) which is connected to this point via a lead 78 is, therefore, not affected by dust accumulation.

Although the resistance proportions between the measurement chamber 68 and the reference resistor remain constant, the dust accumulation on the source of radioactive radiation 72 causes in the represented example. of embodiment a reduction of the ionization current, which can finally become excessive. In order to detect this reduction, produce a dust alarm signal and, if desired, prevent a fire alarm signal the means which have already been described as for example in accordance with FIG. 9 can be used.

In the ionization fire detector in accordance with FIG. 11 there are again connected in series between the positive clamp 9 and the negative clamp 10 a measurement chamber 79 and the reference resistor in the form of the reverse-biased diode 61. The measurement chamber 79 has an outer electrode 80, an inner electrode 81, a measurement space 82 formed between them and a source of radioactive radiation 83. The arrangement is mainly characterized in that within the measurement chamber 83 an additional auxiliary electrode is held by an insulator in spaced relation to an electrode contacting the insulator in such a way that in the case of dust accumulation on the insulator the insulating resistance between the auxiliary electrode and the electrode contacting the insulator decreases, whereas the insulating resistance between the auxiliary electrode and the electrode which does not directly contact the insulator remains practically constant, the auxiliary electrode being connected to a potential different to that of the electrode contacting the insulator, and the leakage current flowing via the insulating resistance serving as the parameter characteristic of the dust accumulation on the radioactive source. In the case of the present example this is achieved by the fact that the auxiliary electrode is formed by the source of radioactive radiation 83 which is held within a ring 84 of insulating plastic material in the outer electrode which has a corresponding circular opening. On the one hand, the source of radioactive radiation 83 which serves as an auxiliary electrode is connected to the positive clamp 9 via the leakage resistance on the surface of the ring 84 which decreases in the case of dust accumulation. The action of the leakage resistance is illustrated by a resistor 85(actually not existing) represented in hatched form, and connected between the source of radioactive radiation 83 and the clamp 9. On the other hand, the source of radioactive radiation 83 -is connected to the negative clamp 10 via a high-ohmic resistor in the form of a reverse-biased diode 86. The potential at a point between the source of radioactive radiation 83 and the diode 86 is, therefore, dependent on the leakage resistance of ring 84 as represented by resistance 85; with decreasing leakage resistance the potential becomes increasingly positive. This change of the potential which is caused by dust accumulation on the ring 84 also indicates a corresponding dust accumulation on the source of radioactive radiation 83. By means of a field-effect transistor 87, which is connected to the point between the source of radioactive radiation 83 and the diode 86, and an alarm producing means such as the threshold amplifier 55 of FIGS. 6 and 8 connected to the output of field-effect transistor 87, there can be produced a dust alarm signal in the case of excessive dust collection on the source of radioactive radiation 83, and/or a fire alarm can be prevented.

In the ionization fire detector in accordance with FIG. 12 the measurement chamber 79 is arranged in the same way as in the one in accordance to FIG. 11, and the source of radioactive radiation 83 is connected via the leakage resistance of ring 85 as represented by resistance 85, to the positive clamp 9 and, via the reverse-biased diode 86, to the negative clamp 10. The reference resistor connected in series with measurement chamber 79 is in this case, however, formed by a diode 61 and a transistor 76, as in the example of embodiment in accordance with FIG. 9. With increasing dust accumulation on the source of radioactive radiation 83, ionization current and leakage resistance of ring 84 decreasing accordingly, a controller 88 which is connected to a point between the source of radioactive radiation 83 and the diode 86 controls the conductibility of the transistor 76 in such a way that the resistance proportions between the internal resistance of the measurement chamber 79 and the reference resistor are kept constant so that an alarm producing means which is connected to a point between the measurement chamber 79 and the diode 61 via lead 89 is practically not influenced.

The measures in accordance with FIGS. 11 and 12 can be applied in combination.

The ionization fire detector represented in FIG. 13 is characterized in that the ionization chamber (measurement chamber) is provided with an additional auxiliary electrode, that in the quiescent state the auxiliary electrode and an adjacent electrode have equal potential and are arranged in such a way that partial currents of the ionization current flow through them, that the difference of the partial currents is measured and that in the case of a reduction of the ionization current below a threshold value suited for signalling a fire, a fire alarm signal is only produced if said difference surpasses a certain value. In the represented case the partial currents of the ionization current are equal in the quiescent state and a fire alarm signal is only produced if the difference between both partial currents is different from zero.

As represented, a measurement chamber 90 is provided, which has an outer electrode 91, an inner electrode 92, a measurement space 93 formed between them, and a source of radioactive radiation 94. The inner electrode 92 is connected in series with a reference resistor in the form of a reverse-biased diode 61 A between the positive clamp 9 and the negative clamp 10. In addition to the inner electrode 92 which has the form ofa circular disk there is provided an annular auxiliary electrode 95 surrounding it concentrically and being connected to the negative clamp 10 via a reference resistor in the form of reverse-biased diode 61B, the size and arrangement of the inner electrode 92 and the auxiliary electrode 95 being chosen so that in the quiescent state equal partial currents of the ionization current are flowing through the inner electrode 92 and the auxiliary electrode 95.

To the point between the inner electrode 92 and the diode 61A there is connected a field-effect transistor 32A which is placed in a network between the clamps 9, l and consisting of resistor 33A, potentiometer 34A, Zener diode 35A and another resistor 36A. This network acts in the same way as the one inaccordance with FIG. which consists of field-effect transistor 32, resistor 33, potentiometer 34, Zener diode 35 and the further resistor 36. To the drain electrode of the fieldeffect transistor 32A there is connected a threshold amplifier 96 which produces at its output connected to clamp 48 a fire alarm signal when the partial current of the ionization current flowing via the inner electrode I 92 is either reduced or increased beyond. a respective threshold value by smoke or ions entering the measurement space.

To a point between auxiliary electrode 95 and the diode 61B there is connected a field-effect transistor 32B, which is placed in a network connected between clamps 9, 10 and consistingof a resistor 333, a potentiometer 348, a Zener diode 35B and another resistor 363. The arrangement of this network is exactly the same as with the network associated with field-effect transistor 32A. The voltages at the drain electrodes of the field-effect transistors 32A, 32B are, therefore, always equal to each other in the quiescent state and also in the case of dust collection on the source of radioactive radiation 94. A differential amplifier 97 which is connected to the output of field-effect transistors 32A, 32B determine the difference between these voltages and prevents the threshold value amplifier to produce a fire alarm signal when said difference is zero. Therefore, no fire alarm is produced in the case of dust accumulation on the source of radiation 94 in spite of a reduction of the ionization current.

When smoke aerosols or ions enter into the measurement chamber 90 these show at least initially different volume concentration values in different parts of the measurement space 82. Therefore, different partial currents of the now modified ionization current are flowing via the inner electrode 92 and the auxiliary electrode 95. Because of the difference between the two partial currents being now different from zero, as detected by the differentiating amplifier 97, the threshold amplifier 96 is now enabled to produce a fire alarm signal.

In order to maximize the effect that in the case of a fire the partial currents of the ionization current become different, it is of advantage, if the inner electrode 92 and the auxiliary electrode 95 have a different distance from the air entry apertures (not shown) which are provided in the measurement chamber 90. This is achieved, for example, by the indicated concentric arrangement of the inner electrode 92 and auxiliary electrode 95 surrounding it.

Those parts of the examples of embodiment which have not been mentioned correspond to the parts of the other examples according to their reference numerals.

While embodiments of the invention have been shown in detail it will be obvious to those skilled in the art that the invention may be embodied otherwise without departing from its spirit and scope.

What is claimed is:

l. A fire detecting apparatus, comprising structural means forming a chamber and including a pair of electrode means, said chamber having gas access openings, and ionization current generating means for generating a quiescent ionization current between said electrode means in the absence of heat emitted gases so that gases from smokeless fires passing through the access openings produce an added ionization current, said generating means including circuit means connected to said electrode means for producing a signal indicative of the state of the ionization current and radioactive source means located in the chamber and having a radioactivity sufficient to limit the quiescent ionization to the order of magnitude of the added ionization current, said radioactive source means including a substance ,having a radioactivity up to 0.1 microcuries, said circuit means including amplifying means for amplifying decreases as well as increases in the potential difference between said pair of electrode means.

2. An apparatus as in claim 1, wherein said radioactive source means includes a substance having a radioactivity of less than 0.05 microcuries.

3. An apparatus as in claim 1, wherein one of said electrode means partially surrounds the other so as to form inner and outer electrode means.

4. An apparatus as in claim 3, wherein said circuit means, when'energized, renders the potential of said inner electrode means more negative than said outer electrode means.

5. An apparatus as in claim 4, wherein said circuit means includes amplifying means having an input connected to said more negative one of said electrode means.

6. An apparatus as in claim 1, wherein said circuit means includes amplifier means connected across said electrode means and including a field-effect transistor for amplifying potential differences between said electrode means.

7. An apparatus as in claim 1, wherein said circuit means includes second structural means forming a reference ionization chamber and second a pair of electrode means, second radioactive source means in said reference ionization chamber, said circuit means connecting said pairs of electrode means in series and forming with said second source means ionization current generating means for producing a reference quiescent ionization current, said reference chamber means being protected from ambient air.

8. An apparatus as in claim 4, wherein said sources means is arranged in said chamber so that its radiation is in the direction of the potential which is negative with respect to the ions generated by it.

9. An apparatus as in claim 4, wherein said source means is mounted on the less negative one of said electrode means.

10. An apparatus as in claim 1, wherein said circuit means includes first amplifying means for responding to decreases in potentials between said electrode.

means and second amplifying means for responding to increases in the potential difference between said electrode means.

11. An apparatus as in claim 10, wherein each of said amplifying means includes field-effect transistors.

12. An apparatus as in claim 1 ll, wherein each of said amplifying means includes two cascade connected field-effect transistors.

13. An apparatus as in claim 7, wherein said circuit means includes a bridge with each of said pair of electrode means forming an arm of said bridge adjacent the other, and wherein amplifying means in said circuit means indicates unbalance in said bridge.

14. An apparatus as in claim 1, wherein said circuit means includes time responsive means for responding to the change in ionization currents with respect to time, and threshold means responsive to said time responsive means for producing a first output in response to a change in ionization current less than a predetermined rate and a second output in response to a change in predetermined value greater than a predetermined rate.

15. An apparatus as in claim 1, wherein said circuit means includes limit means responsive to the ionization current for producing one indication when the ionization current is below a predetermined value and a second indication when the ionization exceeds the predetermined value.

16. An apparatus as in claim 1, wherein said circuit means includes dust sensing means in the chamber, and control means responsive to said sensing means for varying the ionization current in response to said dust sensing means.

17. An apparatus as in claim 14, wherein said time responsive means includes a differentiating network.

18. An apparatusas in claim 15, wherein said circuit means includes time responsive means for responding to the change in ionization currents with respect to time, and threshold means responsive to said time responsive means for producing a first output in response to a change in ionization current less than a predetermined rate and a second output in response to a change in predetermined value greater than a predetermined rate.

19. An apparatus as in claim 18, said circuit means includes dust sensing means in the chamber, and control means responsive to said sensing means for varying the ionization current in response to said dust sensing means.

20. An apparatus as in claim 16, wherein said circuit means includes limit means responsive to the ionization current for producing one indication when the ionization current is. below a predetermined value and a second indication when the ionization exceeds the predetermined value.

21. An apparatus as in claim 16, wherein said sensing means includes a radiation detector in the chamber and exposed to radiation from the radioactive source means so as to have the output vary in response to dust accumulation in the chamber.

22. An apparatus as in claim 16, wherein said sensing means includes auxiliary electrode means mounted in said chamber and insulating means separating said auxiliary electrode means from said pair of electrode means so that dust varies the resistance between said auxiliary electrode means and said one of said pair of electrode means.

23. An apparatus as in claim 22, wherein said auxiliary electrode means includes said radioactive source.

24. An apparatus as in claim 22, wherein said circuit means connects said auxiliary electrode means and one of said pair of electrode means so as to exhibit equal potentials, and wherein said circuit means measures the difference in partial currents through said auxiliary electrode means and one of said pair of electrode means so as to produce a first indication when the difference between the partial currents exceeds a certain value and a second indication when the difference in partial currents is less than the certain value.

25. An apparatus as in claim 24, wherein the threshold value is a difference of the two partial currents 

1. A fire detecting apparatus, comprising structural means forming a chamber and including a pair of electrode means, said chamber having gas access openings, and ionization current generating means for generating a quiescent ionization current between said electrode means in the absence of heat emitted gases so that gases from smokeless fires passing through the access openings produce an added ionization current, said generating means including circuit means connected to said electrode means for producing a signal indicative of the state of the ionization current and radioactive source means located in the chamber and having a radioactivity sufficient to limit the quiescent ionization to the order of magnitude of the added ionization current, said radioactive source means including a substance having a radioactivity up to 0.1 microcuries, said circuit means including amplifying means for amplifying decreases as well as increases in the potential difference between said pair of electrode means.
 2. An apparatus as in claim 1, wherein said radioactive source means includes a substance having a radioactivity of less than 0.05 microcuries.
 3. An apparatus as in claim 1, wherein one of said electrode means partially surrounds the other so as to form inner and outer electrode means.
 4. An apparatus as in claim 3, wherein said circuit means, when energized, renders the potential of said inner electrode means more negative than said outer electrode means.
 5. An apparatus as in claim 4, wherein said circuit means includes amplifying means having an input connected to said more negative one of said electrode means.
 6. An apparatus as in claim 1, wherein said circuit means includes amplifier means connected across said electrode means and including a field-effect transistor for amplifying potential differences between said electrode means.
 7. An apparatus as in claim 1, wherein said circuit means includes second structural Means forming a reference ionization chamber and second a pair of electrode means, second radioactive source means in said reference ionization chamber, said circuit means connecting said pairs of electrode means in series and forming with said second source means ionization current generating means for producing a reference quiescent ionization current, said reference chamber means being protected from ambient air.
 8. An apparatus as in claim 4, wherein said sources means is arranged in said chamber so that its radiation is in the direction of the potential which is negative with respect to the ions generated by it.
 9. An apparatus as in claim 4, wherein said source means is mounted on the less negative one of said electrode means.
 10. An apparatus as in claim 1, wherein said circuit means includes first amplifying means for responding to decreases in potentials between said electrode means and second amplifying means for responding to increases in the potential difference between said electrode means.
 11. An apparatus as in claim 10, wherein each of said amplifying means includes field-effect transistors.
 12. An apparatus as in claim 11, wherein each of said amplifying means includes two cascade connected field-effect transistors.
 13. An apparatus as in claim 7, wherein said circuit means includes a bridge with each of said pair of electrode means forming an arm of said bridge adjacent the other, and wherein amplifying means in said circuit means indicates unbalance in said bridge.
 14. An apparatus as in claim 1, wherein said circuit means includes time responsive means for responding to the change in ionization currents with respect to time, and threshold means responsive to said time responsive means for producing a first output in response to a change in ionization current less than a predetermined rate and a second output in response to a change in predetermined value greater than a predetermined rate.
 15. An apparatus as in claim 1, wherein said circuit means includes limit means responsive to the ionization current for producing one indication when the ionization current is below a predetermined value and a second indication when the ionization exceeds the predetermined value.
 16. An apparatus as in claim 1, wherein said circuit means includes dust sensing means in the chamber, and control means responsive to said sensing means for varying the ionization current in response to said dust sensing means.
 17. An apparatus as in claim 14, wherein said time responsive means includes a differentiating network.
 18. An apparatus as in claim 15, wherein said circuit means includes time responsive means for responding to the change in ionization currents with respect to time, and threshold means responsive to said time responsive means for producing a first output in response to a change in ionization current less than a predetermined rate and a second output in response to a change in predetermined value greater than a predetermined rate.
 19. An apparatus as in claim 18, said circuit means includes dust sensing means in the chamber, and control means responsive to said sensing means for varying the ionization current in response to said dust sensing means.
 20. An apparatus as in claim 16, wherein said circuit means includes limit means responsive to the ionization current for producing one indication when the ionization current is below a predetermined value and a second indication when the ionization exceeds the predetermined value.
 21. An apparatus as in claim 16, wherein said sensing means includes a radiation detector in the chamber and exposed to radiation from the radioactive source means so as to have the output vary in response to dust accumulation in the chamber.
 22. An apparatus as in claim 16, wherein said sensing means includes auxiliary electrode means mounted in said chamber and insulating means separating said auxiliary electrode means from said pair of electrode means so that dust varies the resistance between said auxiliary electrode means and said one of said pair of electrode means.
 23. An apparatus as in claim 22, wherein said auxiliary electrode means includes said radioactive source.
 24. An apparatus as in claim 22, wherein said circuit means connects said auxiliary electrode means and one of said pair of electrode means so as to exhibit equal potentials, and wherein said circuit means measures the difference in partial currents through said auxiliary electrode means and one of said pair of electrode means so as to produce a first indication when the difference between the partial currents exceeds a certain value and a second indication when the difference in partial currents is less than the certain value.
 25. An apparatus as in claim 24, wherein the threshold value is a difference of the two partial currents equal to zero. 